I. Field of the Invention
This invention relates to an alkaline blast cleaning system for aluminum surfaces which minimizes or eliminates discoloring or tarnishing of the aluminum surfaces. The invention further relates to processes for using the system in cleaning aluminum surfaces without causing significant discoloring or tarnishing of the aluminum.
More specifically, the invention concerns the use of small amounts of an alkali metal silicate, preferably sodium silicate, in conjunction with alkali metal bicarbonates, particularly sodium bicarbonate, in blast cleaning systems to substantially reduce or altogether prevent alkali attack on aluminum, particularly aircraft aluminum. The present invention also particularly relates to an improved method for cleaning or stripping paint from the exterior surface of aircraft by blasting sodium bicarbonate inhibited with sodium silicate against the aircraft's exterior surfaces.
II. The Prior Art
Until recently, stripping of paint from the exterior surfaces of airplanes was accomplished by use of methylene chloride or formic acid stripping products or solutions. In practice, the airplane would be wheeled into a hangar, and the door to the hangar closed. The workers would don respirators and rubber protective suits and gloves. Then the workers would coat the painted aircraft surfaces with the stripper and subsequently remove the residue with a solvent and repeat the steps as necessary. Upon completion of the paint stripping, the stripped paint and solvent residue would be hosed down the floor drain. Obviously, the whole procedure was hazardous to the workers and the environment. Many states are considering the banning of chemical stripping, leaving sanding by hand as the only approved method for removing paint from airplanes.
Then, it was proposed that sodium bicarbonate be blasted against the painted surfaces by means of pressurized air in order to strip the paint. Although that process avoided the use of ecologically undesirable solvents, the new process produced undesirable clouds of sodium bicarbonate dust. Therefore, it was proposed that a water spray be used with the sodium bicarbonate blasting in order to reduce or eliminate the clouds of sodium bicarbonate dust. We have now found that the system may be inhibited against discoloration or corrosion of aluminum with aqueous sodium silicate solution.
Sodium bicarbonate itself is relatively benign to aircraft aluminum. However, copper-containing alloys of aluminum may darken on contact with bicarbonate/carbonate solutions. Some experts have evaluated the darkening and have the perception that it is the result of the formation of a protective oxide coating, and may well be beneficial. Others in the aircraft industry and among the air fleet owners view the darkening phenomenon as a significant aesthetic or potential corrosion problem. Consequently, we designed our corrosion-inhibiting cleaning systems and process for using them to eliminate or effectively inhibit the possible discoloration problem.
In searching for a way to inhibit or eliminate the potential corrosion problem discussed above, we considered a number of candidate inhibitors. Although potentially effective, many were rejected because of ecological hazards they posed--e.g., chromates. Other inhibitor candidates were used in corrosion tests and found wanting. Upon the completion of our research, we determined that aqueous solutions of sodium silicate at certain concentrations were, surprisingly, the best inhibitor.
It is generally known to treat metal surfaces, e.g., aluminum surfaces, with an aqueous solution of alkali metal silicates, e.g., water glass. The treatments, which include cleaning and/or coating etc., have been done with water glass alone (see, for example, U.S. Pat. Nos. 4,457,322 and 4,528,039) or in conjunction with one or more additives depending on the purpose of the treatment. The patents, which teach the use of one or more additives with the water glass, do not, however, teach the use of bicarbonates in conjunction with the water glass as disclosed in this invention.
Although some patents teach the use of water glass to treat aluminum, none is known which discloses the combination of water glass and bicarbonates, either as a composition, e.g., blast medium, or in a method, to treat aluminum as set forth in this invention.
Rubin et al. U.S. Pat. Nos. 4,457,322 and 4,528,039 disclose that water glass (sodium silicate) alone has been widely used in treating aluminum surfaces. They indicate that a limitation of such a treatment is the inability of water glass to remove certain deposits, due to its low alkalinity. The process proposed to overcome the problem employs an aqueous mixture of an alkali metal metasilicate with sodium-, potassium-, or lithium carbonate, potassium- or sodium orthophosphates or mixtures thereof.
Rubin et al. recognize, as we have found, that certain compositions, e.g., carbonates or orthophosphates, damage and discolor aluminum (see Examples 1,2, 4, 6, 7 and 8). They teach that small concentrations of metasilicate minimizes or prevents their attack on aluminum metal surfaces. The alkali metal carbonates are the only carbonates considered and bicarbonates are not disclosed.
Easton U.S. Pat. No. 4,125,969 is concerned with the wet abrasion blast cleaning of a metallic surface using powdered sodium silicate (water glass) as the abrasive material. The sodium silicate is only partially solubilized when applied, the particulate portion providing the abrasive action. Easton discloses that other active materials may be used with the sodium silicate, e.g., rust inhibitors for ferrous surfaces, etching agents, or certain "surface protection composition" which may be in solution when combined with the sodium silicate. Bicarbonates are not disclosed, however. While the treatment of metal surfaces is discussed, aluminum is not specifically mentioned.
The following three patents teach the use of alkali metal silicates in combination with other components to treat aluminum surfaces.
Seidl U.S. Pat. No. 2,978,361 discloses the use of an alkali metal silicate, e.g., water glass, and at least one other metal, either partially or wholly in the form of its silicate, to coat a metal surface. The coating is especially effective when sprayed on a metal surface which has a high affinity for oxygen, e.g., aluminum.
Duval et al. U.S. Pat. No. 3,458,300 discloses the treatment of aluminum surfaces, e.g., aircraft skin, with a combination of sodium metasilicate with aluminum oxide and a wetting agent.
Etherington et al. U.S. Pat. No. 3,499,780 teaches coating an aluminum substrate, after a brightening step, with a solution comprising an alkali metal silicate, e.g., water glass, and then baking the coating to harden it.
Although the above patents disclose the combination of various agents with water glass, none teaches the use of bicarbonates.
Three patents also disclose the treatment of metal surfaces with alkali metal silicates in combination with other additives. Aluminum surfaces, however, are not specifically referred to. See Curtin U.S. Pat. No. 2,816,195, Ryznar U.S. Pat. No. 3,037,866 and Uhlmann U.S. Pat. No. 3,544,366.
A number of patents disclose the use of mixtures of water glass with sodium bicarbonates, but none is concerned with the treatment of metal, especially aluminum surfaces. See, for example, Imschenetzky U.S. Pat. No. 631,719, Lathe et al. U.S. Pat. No. 2,218,244 and Payne U.S. Pat. No. 4,552,804.
The object of the present invention is to provide a simple but effective corrosion-inhibited blasting means and process for cleaning aluminum surfaces, particularly the aluminum surfaces of airplanes. It is an object of the invention to provide an inhibitor for the blasting media that will reduce the corrosion rate of carbonates on aircraft aluminum to less than that of distilled water. It is another object of the invention to provide an inhibitor for the blasting media that will be safe to handle. It is a still further object of the invention to provide an inhibitor for the blasting media that will be ecologically safe.